A conventional cordless window covering system includes a headrail, a bottom rail, a covering material and a driving assembly, wherein the driving assembly is usually a spring box. The covering material is disposed between the headrail and the bottom rail, wherein the covering material can be expanded or collected below the headrail while the bottom rail ascends or descends. During the collection of the covering material, the spring box provides a necessary driving force for the covering material to be collected, wherein the driving force is accumulated in the spring box during the expansion of the covering material. A retaining position of the covering material during collection and expansion is controlled by a balance between the driving force of the spring box, a friction force of the window covering system and an overall weight of the bottom rail and the covering material. In other words, when the weight of the covering material and the bottom rail is balanced against the friction force of the window covering system, the bottom rail stops at the retaining position such that light blockage of the covering material is maintained. However, it is difficult to control the overall friction force of the window covering system effectively comparing to the weight of the bottom rail and the covering material. In addition, the spring box provides the driving force by a spiral spring, which has a potential problem of losing elasticity due to fatigue. Moreover, the closer the bottom rail ascends to the headrail, the more covering material accumulates on the bottom rail, and hence the heavier the overall weight of the bottom rail and the covering material. Therefore, it is likely that the bottom rail would more or less descend for a distance, which is undesired, from a desired retaining position. In such case, it is inconvenient and annoying to anyone operating the window covering system.
In order to solve the above-mentioned problem, the window covering system may further include a retaining assembly to hold the bottom rail at the retaining position; and therefore the problem of undesired descending of the bottom rail due to gravity can be solved. However, a releasing assembly corresponding to the retaining assembly is needed to release the bottom rail. Since the reachable height of users varies, it is inconvenient when the bottom rail or the releasing assembly is at a position beyond users' reach. In addition, when the bottom rail is released from the retaining assembly, the covering material may expand rapidly to cause damage to the mechanical components of the window covering system or hurt the users.